Semiconductor capacitor and power supply module

ABSTRACT

A semiconductor capacitor includes a semiconductor substrate having a first and second principal surfaces. A first set of one or more trenches is formed on the first principal surface and a second set of one or more trenches formed on the second principal surface. A first dielectric film is located on the first principal surface and least inner walls of the first set of one or more trenches. A second dielectric film is located on the second principal surface and least inner walls of the second set of one or more trenches. A first conductor film located on the first dielectric film. A second conductor film located on the second dielectric film. The semiconductor substrate is formed of Si, SiC, GaN, or the like. The dielectric film has a two-layer structure of SiO 2  and Si 3 N 4 .

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of International applicationNo. PCT/JP2016/088638, filed Dec. 26, 2016, which claims priority toJapanese Patent Application No. 2016-030684, filed Feb. 22, 2016, theentire contents of each of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a semiconductor capacitor, and moreparticularly, to a semiconductor capacitor with a high voltageendurance.

The present invention also relates to a power supply module includingthe semiconductor capacitor of the present invention.

BACKGROUND OF THE INVENTION

Semiconductor capacitors including a semiconductor substrate are usedfor on-vehicle devices, industrial devices, and the like as capacitorsexcellent in both voltage endurance and heat resistance.

One such semiconductor capacitor is disclosed in Japanese PatentApplication Laid-Open No. 2006-261416. FIG. 5 shows a semiconductorcapacitor 1000 disclosed in the foregoing application. As shown therein,the semiconductor capacitor 1000 includes a semiconductor substrate 101,formed, for example, of Si (silicon). A plurality of trenches 102 areformed on a first principal surface (the upper principal surface asviewed in FIG. 5) of the semiconductor substrate 101.

A dielectric film (insulating film) 103 is formed on an inner wall ofeach of the trenches 102. The dielectric film 103 is formed of, forexample, an oxide film. That is, the dielectric film 103 is formed of,for example, a silicon oxide film formed by oxidizing the surface of thesemiconductor substrate 101.

A conductor film (electrode) 104 is formed on the dielectric film 103.The conductor film 104 is made of, for example, a metal or polysilicondoped with impurities or the like.

Semiconductor capacitor 1000 has a structure excellent in voltageendurance and includes a material excellent in voltage endurance, andtherefore it has high voltage endurance. However, it uses a materialhaving resistance to heat and therefore it has high heat resistance.

A semiconductor capacitor has high voltage endurance and high heatresistance as described above. However, a further increase in withstandvoltage of the semiconductor capacitor has been required in variousdevices using the semiconductor capacitor. In such devices, it isadvantageous for the device to be driven at as high a voltage aspossible since the power loss is reduced. For example, the semiconductorcapacitor may be used as a snubber capacitor for a power supply modulesuch as an on-vehicle device, but in an on-vehicle device or the like,it is required to be driven at a voltage as high as possible in order tosuppress depletion of a battery as much as possible. With an increasesin a voltage of the device, a further increase in withstand voltage hasbeen required for the semiconductor capacitor to be used.

In order to enhance the voltage endurance of the semiconductorcapacitor, it is known to increase the thickness of the dielectric film.However, when the thickness of the dielectric film is increased, theintensity of electric field concentration to an end portion of theconductor film (electrode) formed on the dielectric film also increasesalong with the increase in dielectric film thickness, and therefore thevoltage endurance is not significantly improved. For example, even ifthe thickness of the dielectric film is doubled, the voltage enduranceis improved only by, at most, about 1.5 times.

Further, in order to increase the thickness of the dielectric film inthe semiconductor capacitor, there is a problem that the time requiredfor forming the dielectric film is prolonged in the manufacturingprocess of the semiconductor capacitor.

BRIEF DESCRIPTION OF THE INVENTION

The present invention has been made to solve the above-mentionedconventional problems.

In accordance with one aspect of the invention, a semiconductorcapacitor includes a semiconductor substrate having a first and secondprincipal surfaces. A first set of one or more trenches is formed on thefirst principal surface and a second set of one or more trenches formedon the second principal surface. A first dielectric film is located onthe first principal surface and least inner walls of the first set ofone or more trenches. A second dielectric film is located on the secondprincipal surface and least inner walls of the second set of one or moretrenches. A first conductor film located on the first dielectric film. Asecond conductor film located on the second dielectric film.

The semiconductor substrate is preferably made of any one of Si, SiC andGaN and preferably includes a plurality of layers. In one aspect of theinvention, each of the dielectric films comprises a first layer made ofSiO₂ and the second layer made of Si₃N₄.

In a preferred embodiment, the first and second principal surfacesoppose one another. More particularly, they are parallel to and spacedfrom one another.

In a preferred embodiment, the first and second dielectric films coverthe entire first and second principal surfaces, respectively. In a morepreferred embodiment, the first dielectric film covers all inner wallsof the first set of one or more trenches and the second dielectric filmcovers all inner walls of the second set of one or more trenches.

In the preferred embodiment, first and second external electrodes arelocated on the first and second conductive films, respectively. In apreferred embodiment the first and second conductor films and the firstand second external electrodes do not extend to outer peripheral edgesof the semiconductor capacitor and respective insulating films arelocated on the first and second dielectric films, respectively atlocations between the first and second external electrodes and the outerperipheral edges of the semiconductor capacitor, respectively.

In another aspect of the invention, the invention is directed towards apower supply module comprising a semiconductor switching element and thecapacitor described above connected in parallel to the semiconductorswitching element.

Since the semiconductor capacitor of the present invention has highvoltage endurance, in this case, a voltage of the power supply modulecan be increased and the power loss of the power supply module can bereduced. In addition, since the semiconductor capacitor has high heatresistance, it can be arranged in the immediate vicinity of asemiconductor switching element that generates high heat. As a result,since a wiring of the semiconductor capacitor can be shortened, thepower supply module can be reduced in equivalent series inductance(ESL).

In the semiconductor capacitor of the present invention, a trench isformed on each of a first principal surface and a second principalsurface of a semiconductor substrate, and a dielectric film and aconductor film are formed on an inner wall of the trench, so that thesemiconductor capacitor has an equivalent circuit in which twocapacitors are connected in series, and therefore it has high voltageendurance.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a semiconductor capacitor 100according to a first embodiment of the invention.

FIG. 2(A) to FIG. 2(F) are cross-sectional views showing each of stepsperformed in an example of a manufacturing method of the semiconductorcapacitor 100.

FIG. 3(A) is a cross-sectional view showing a semiconductor capacitor1100 according to Comparative Example 1.

FIG. 3(B) is a cross-sectional view showing a semiconductor capacitor1200 according to Comparative Example 2.

FIG. 4 is an equivalent circuit diagram showing a power supply module200 according to a second embodiment.

FIG. 5 is a cross-sectional view showing a conventional semiconductorcapacitor 1000 disclosed in Patent Document 1 (Japanese PatentApplication Laid-Open No. 2006-261416).

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments for carrying out the present invention will bedescribed with reference to the drawings.

Each embodiment illustratively shows embodiments of the presentinvention, and the present invention is not limited to the descriptionof the embodiments. Further, the drawings are for helping theunderstanding of the embodiments, and there may be cases where thedrawings are not necessarily drawn strictly. For example, the ratio ofthe sizes of a constituent element or between constituent elements drawnmay not correspond to the ratios of those dimensions as described in thespecification. In addition, the constituent elements described in thespecification may be omitted in the drawings, or the number of theconstituent elements drawn may be reduced, and the like.

First Embodiment

FIG. 1 shows a cross-sectional view of a semiconductor capacitor 100according to a first embodiment of the present invention. Thesemiconductor capacitor 100 includes a semiconductor substrate 1 made,for example, of Si. However, a material of the semiconductor substrate 1is optional and may be, for example, SiC, GaN or the like.

The semiconductor substrate 1 has a flat plate shape including a firstprincipal surface (the upper principal surface as viewed in FIG. 1) anda second principal surface (the lower principal surface as viewed inFIG. 1). In the present embodiment, each of the first and secondprincipal surfaces has a rectangular shape. However, the shapes of thefirst and second principal surfaces are not so limited and may be, forexample, a circular shape instead of a rectangular shape.

In the present embodiment, a thickness of the semiconductor substrate 1was set to 600 μm. Further, the sizes in the planar direction were setto 5 mm in length and 5 mm in width. However, the invention is notlimited to these sizes and can be appropriately changed depending on thenumber, depth and the like of a trench 2 to be described later. Theconductivity of the semiconductor substrate 1 is adjusted to a desiredvalue by selecting the material or performing doping as required.

A plurality of trenches 2 are formed on each of the first and secondprincipal surfaces of the semiconductor substrate 1. In the presentembodiment, two-hundred thousand trenches 2 are formed on each of thefirst and second principal surfaces. Each of the trenches 2 has acolumnar (cylindrical) shape having a diameter of 5 μm and a depth of 20μm. However, the number, shape, size, interval, and the like of thetrenches 2 are not so limited and can be appropriately changed inaccordance with a capacity value or the like to be required. Forexample, the shape of the trench 2 may be a polygonal prism shape suchas a quadrangular prism shape, a pentagonal prism shape, a hexagonalprism shape, or the like.

A respective dielectric film 3 is formed on the first and secondprincipal surfaces of the semiconductor substrate including the innerwalls of the trenches 2. Although not shown in the drawing because it isdifficult to see, each dielectric film 3 is formed in a two-layerstructure in the present embodiment.

Each dielectric film includes a first layer made of SiO₂ and a secondlayer made of Si₃N₄. The first (SiO₂) layer has a thickness of 100 nmand is formed by thermally oxidizing Si on the surface of thesemiconductor substrate 1. This layer is provided for enhancing thebonding strength between the semiconductor substrate 1 made of Si andthe second Si₃N₄ layer. The thickness of the second Si₃N₄ layer is 1.1μm and causes the semiconductor capacitor 100 to exhibit a capacitytogether with the first SiO₂ layer.

While the foregoing structure of the dielectric film is one example of adielectric film used in the present invention, the invention is not solimited. Other structures, materials, thicknesses, and the like can beused. For example, the dielectric film 3 may be formed as a single layerstructure made of one type of material.

A respective conductor film 4 is formed on respective dielectric films 3located on the first and second principal surface) of the semiconductorsubstrate 1 (including the inner walls of the trenches 2), respectively.In the present embodiment, the conductor film 4 has a thickness of 500nm and is formed of polysilicon. However, the material, thickness,formation region and the like of the conductor film 4 are not so limitedand can be appropriately changed.

A respective external electrode 5 is formed on respective conductorfilms 4 formed on the first and second principal surfaces (including theinner walls of the trenches), respectively. In the present embodiment,the external electrode 5 has a thickness of 6 μm and is formed of Al.However, the material, thickness, formation region and the like of theexternal electrode 5 are optional and can be appropriately changed.

A respective insulator film 6 is formed around the external electrodes 5formed on the first and second principal surfaces of the semiconductorsubstrate 1, respectively. The insulator film 6 includes a polyimideresin, an epoxy resin, or the like and suppresses the occurrence ofunexpected discharge through an edge portion of the external electrodes5 by covering the edge portion of the external electrodes 5.

The semiconductor capacitor 100 having the above structure according tothe first embodiment has a equivalent circuit in which a capacitor, aresistor (a resistor based on a resistance component of thesemiconductor substrate 1), and a capacitor are connected in seriesbetween a pair of external electrodes 5. The semiconductor capacitor 100has high voltage endurance since two capacitors are connected in series.

The semiconductor capacitor 100 having the above structure according tothe first embodiment can be manufactured, by way of example, inaccordance with the method shown in FIG. 2(A) to FIG. 2(F).

First, as shown in FIG. 2(A), a semiconductor substrate 1 is prepared.Typically, the semiconductor substrate 1 is prepared as a substratewafer (not shown) which includes a plurality of semiconductor substrates1 and is divided into individual products at a predetermined stage inthe manufacturing process. However, in FIG. 2(A) to FIG. 2(F), only onesemiconductor substrate 1 is shown for easy viewing.

Next, as shown in FIG. 2(B), trenches 2 are formed on each of the firstand second principal surfaces of the semiconductor substrate 1. Thetrenches 2 are formed, for example, by using a lithography technique.

Next, as shown in FIG. 2(C), a respective dielectric film 3 is formed oneach of the first and second principal surfaces of the semiconductorsubstrate 1 including on the inner walls of the trenches 2. As describedabove, in the present embodiment, the dielectric film 3 is formed as atwo-layer structure, the first layer being made of SiO₂ and the secondlayer being made of Si₃N₄. First, the first layer made of SiO₂ is formedby thermally oxidizing Si of the surface of the semiconductor substrate1. Subsequently, the second layer made of Si₃N₄ is formed by, forexample, CVD (chemical vapor deposition).

In addition, formation of the first layer made of SiO₂ formed by thermaloxidation and formation of the second layer made of Si₃N₄ formed by CVDcan both be performed simultaneously on the first and second principalsurfaces of the semiconductor substrate 1. In the conventional method ofincreasing the thickness of the dielectric film in order to improve thevoltage endurance described above, there is a problem that the timerequired for forming the dielectric film is prolonged by a time periodcorresponding to the desired increase in the thickness of the dielectricfilm. In contrast, according to the manufacturing method of the presentembodiment, since the dielectric film 3 on the first principal surfaceand the dielectric film 3 on the second principal surface can be formedsimultaneously, the voltage endurance of the semiconductor capacitor 100can be improved without unnecessarily prolonging the manufacturing time.

Next, as shown in FIG. 2(D), respective conductor films 4 are formed onthe respective dielectric films 3 located on the first and secondprincipal surfaces (including the inner walls of the trenches 2) of thesemiconductor substrate 1, respectively. The conductor film 4 is formed,by way of example, using a chemical vapor deposition (CVD) method. TheCVD is typically performed by covering a predetermined region with amask and the conductor film 4 is not formed in the vicinity of the outeredges of the first and second principal surfaces of the semiconductorsubstrate 1.

Next, as shown in FIG. 2(E), a pair of external electrodes 5 are formedon the respective conductor films 4 (including the portions of theconductor films located in the trenches 2). The external electrode 5 maybe formed of, for example, Al.

Next, as shown in FIG. 2(F), respective insulator films 6 are formedaround the external electrodes 5. The insulator films 6 are formed, forexample, of a polyimide resin.

The preceding steps are preferably performed in a state of a substratewafer including a plurality of semiconductor substrates 1, and finally,although not shown, the substrate wafer is divided into individualproducts to complete the semiconductor capacitor 100 according to thefirst embodiment.

[Comparison of Voltage Endurance]

The voltage endurance of the semiconductor capacitor 100 according tothe first embodiment is compared to the voltage endurance of asemiconductor capacitors 1100 according to Comparative Examples 1 and 2,each of which have a conventional structure.

FIG. 3(A) shows a cross sectional view of the semiconductor capacitor1100 of Comparative Example 1. FIG. 3(B) shows a cross sectional view ofthe semiconductor capacitor 1200 of Comparative Example 2.

In the semiconductor capacitor 1100 of Comparative Example 1, trenches12, a dielectric film 13, a conductor film 14, an external electrode 15,and an insulator film 16 are formed only on the first principal surface(the upper principal surface as viewed in FIG. 3(A)). The trenches 12,dielectric film 13, conductor film 14, external electrode 15, andinsulator film 16 of the semiconductor capacitor 1100 are formed in thesame conditions as those of the trenches 2, dielectric film 3, conductorfilm 4, external electrode 5, and insulator film 6 of the semiconductorcapacitor 100 of the first embodiment described above. That is, thedielectric film 13 of the semiconductor capacitor 1100 has the samestructure, material and thickness as those of the dielectric film 3 ofthe semiconductor capacitor 100. An external electrode 17 for mountingis formed on the second principal surface (the lower principal surfaceas viewed in FIG. 3(A)) of the semiconductor capacitor 1100.

In the semiconductor capacitor 1200 of Comparative Example 2, trenches22, a dielectric film 23, a conductor film 24, an external electrode 25,and an insulator film 26 are formed only on the first principal surface(the upper principal surface as viewed in FIG. 3(B)). The structure andmaterial of the dielectric film 23 of the semiconductor capacitor 1200are the same as the structure and material of the dielectric film 3 ofthe semiconductor capacitor 100 of the first embodiment. That is, thedielectric film 23 of the semiconductor capacitor 1200 is formed in atwo-layer structure in which the first layer is made of SiO₂ and thesecond layer is made of Si₃N₄. However, the thicknesses of the first andsecond layers of the dielectric film 23 of the semiconductor capacitor1200 are respectively formed twice as large as the thicknesses of thefirst layer and second layers of the dielectric film 3 of thesemiconductor capacitor 100 of the above embodiment of the presentinvention. The other constituent elements of the semiconductor capacitor1200, namely, the trench 22, the conductor film 24, the externalelectrode 25, and the insulator film 26 are formed in the sameconditions as those of the trenches 2, dielectric film 3, conductor film4, external electrode 5, and insulator film 6 of the semiconductorcapacitor 100. An external electrode 27 for mounting is formed on thesecond principal surface (the lower principal surface as viewed in FIG.3(B)) of the semiconductor capacitor 1200.

The withstand voltage of the semiconductor capacitor 100 according tothe first embodiment, the withstand voltage of the semiconductorcapacitor 1100 according to Comparative Example 1, and the withstandvoltage of the semiconductor capacitor 1200 according to ComparativeExample 2 are shown in Table 1. However, each withstand voltage is atheoretical value.

TABLE 1 Semiconductor Semiconductor Capacitor 1100 Capacitor 1200Semiconductor according to according to Capacitor 100 ComparativeComparative according to Example 1 Example 2 First Embodiment FormingOnly first Only first First principal Position of principal principalsurface and Dielectric Film surface of surface of second principalsemiconductor semiconductor surface of substrate substrate semiconductorsubstrate Thickness of a a × 2 a Dielectric Film Withstand 100 140-150200 Voltage (V)

In the case where each of the thickness of each of the dielectric films3 of the semiconductor capacitor 100 of the first embodiment and thethickness of the dielectric film 13 of the semiconductor capacitor 1100of Comparative Example 1 is taken as a (μm), the thickness of thedielectric film 23 of the semiconductor capacitor 1200 of ComparativeExample 2 is a ×2 (μm).

Assuming that the withstand voltage of the semiconductor capacitor 1100of Comparative Example 1 is set to 100 (V), the withstand voltage of thesemiconductor capacitor 100 of the first embodiment is 100×2=200 (V).The reason for this is that the semiconductor capacitor 100 isequivalent to connecting of the two semiconductor capacitors 1100 inseries.

In contrast, in the semiconductor capacitor 1200 of Comparative Example2, although the thickness of the dielectric film 23 is twice as large asthe thickness of the dielectric film 13 of the semiconductor capacitor1100 of Comparative Example 1, the withstand voltage is not twice aslarge as that of the semiconductor capacitor 1100 of Comparative Example1 and is improved only to about 140 to 150 (V). That is, at most, thewithstand voltage of the semiconductor capacitor 1200 of ComparativeExample 2 is improved only to about 1.5 times as large as thesemiconductor capacitor 1100 of Comparative Example 1. The reason forthis is that, by increasing the thickness of the dielectric film 23, theintensity of electric field concentration at the end portion of theconductor film 24 indicated by the symbol X in FIG. 4(B) is increased,and as a result, the improvement of the voltage endurance is lessened.

As described above, according to the semiconductor capacitor 100 of thefirst embodiment, it is possible to effectively improve the voltageendurance. In the semiconductor capacitor 100, as described above, sincethe dielectric film 3 on the first principal surface and the dielectricfilm 3 on the second principal surface can be formed simultaneously, themanufacturing time for the semiconductor capacitor 100 is not prolongedin order to improve the voltage endurance.

Second Embodiment

FIG. 4 is an equivalent circuit diagram of a power supply module 200according to a second embodiment of the present invention. The powersupply module 200 is prepared using the semiconductor capacitor 100according to the first embodiment.

The power supply module 200 includes a P-side terminal P, an N-sideterminal N, and an intermediate terminal U. Two semiconductor switchingelements S1, S2 are connected in series between the P-side terminal Pand the N-side terminal N. Furthermore, a connection point between thesemiconductor switching element S1 and the semiconductor switchingelement S2 is connected to the intermediate terminal U.

In the power supply module 200, a respective semiconductor capacitor 100according to the first embodiment is connected in parallel to each ofthe semiconductor switching elements S1 and S2. Each semiconductorcapacitor 100 is connected as a snubber capacitor and plays a role ofreducing parasitic inductance or parasitic resistance generated inwiring, wire bonding and the like at the time of switching of thesemiconductor switching elements S1 and S2 to lower noise.

In FIG. 4, each semiconductor capacitor 100 is represented by anequivalent circuit in which a capacitor, a resistor, and a capacitor areconnected in series. The two capacitors represent a first capacitorformed on the first principal surface of the semiconductor substrate 1and a second capacitor formed on the second principal surface. Theresistor represents resistance due to the resistance component of thesemiconductor substrate 1.

In the power supply module 200, semiconductor switching elements S1 andS2 generate extremely high temperature heat. However, since thesemiconductor capacitor 100 according to the first embodiment has highheat resistance, a respective semiconductor capacitor 100 can bearranged in the immediate vicinity of the semiconductor switchingelements S1 and S2. As a result, the wiring of the semiconductorcapacitor 100 can be shortened and the ESR of the power supply module200 can be lowered.

In addition, since the semiconductor capacitor 100 according to thefirst embodiment, which is used for the power supply module 200, has ahigh withstand voltage, the voltage of the power supply module 200 canbe set high. As a result, the power supply module 200 has low power lossand high efficiency.

The semiconductor capacitor 100 according to the first embodiment andthe power supply module 200 according to the second embodiment have beendescribed above. However, the present invention is not limited to thedescription above, and various modifications can be made in accordancewith the purport of the invention.

For example, in the case of the semiconductor capacitor, the material,shape, and size of each constituent element, and the number of theconstituent elements, and the like are optional, and are not limited tothe description above.

In the power supply module, the equivalent circuit is optional, and isnot limited to the description above.

DESCRIPTION OF REFERENCE SYMBOLS

-   1: Semiconductor substrate-   2: Trench-   3: Dielectric film-   4: Conductor film-   5: External electrode-   6: Insulator film-   100: Semiconductor capacitor-   S1, S2: Semiconductor switching element-   200: Power supply module

The invention claimed is:
 1. A semiconductor capacitor comprising: asemiconductor substrate having a first and second principal surfaces; afirst set of one or more trenches formed on the first principal surface;a second set of one or more trenches formed on the second principalsurface; a first dielectric film located on the first principal surfaceand least inner walls of the first set of one or more trenches, thefirst dielectric film covering the entire first principal surface andall inner walls of the first set of one or more trenches; a seconddielectric film located on the second principal surface and least innerwalls of the second set of one or more trenches, the second dielectricfilm covering the entire second principal surface and all inner walls ofthe second set of one or more trenches; a first conductor film locatedon the first dielectric film; a second conductor film located on thesecond dielectric film; and first and second external electrodes locatedon the first and second conductive films, respectively; wherein thefirst and second conductor films and the first and second externalelectrodes do not extend to outer peripheral edges of the semiconductorcapacitor.
 2. The semiconductor capacitor according to claim 1, whereinthe semiconductor substrate is made of any one of Si, SiC and GaN. 3.The semiconductor capacitor according to claim 1, wherein each of thedielectric films comprises a plurality of layers.
 4. The semiconductorcapacitor according to claim 1, wherein each of the dielectric filmscomprises a first layer made of SiO₂ and the second layer made of Si₃N₄.5. The semiconductor capacitor according to claim 1, wherein the firstand second principal surfaces oppose one another.
 6. The semiconductorcapacitor according to claim 5, wherein the first and second principalsurfaces are parallel to and spaced from one another.
 7. Thesemiconductor capacitor according to claim 1, further comprisingrespective insulating films located on the first and second dielectricfilms, respectively and at locations between the first and secondexternal electrodes and the outer peripheral edges of the semiconductorcapacitor, respectively.
 8. A power supply module comprising: asemiconductor switching element; and a capacitor in accordance withclaim 1 connected in parallel to the semiconductor switching element. 9.The power supply module according to claim 8, wherein the semiconductorsubstrate is made of any one of Si, SiC and GaN.
 10. The power supplymodule according to claim 8, wherein each of the dielectric filmscomprises a plurality of layers.
 11. The power supply module accordingto claim 8, wherein each of the dielectric films comprises a first layermade of SiO₂ and the second layer made of Si₃N₄.
 12. The power supplymodule according to claim 8, wherein the first and second principalsurfaces oppose one another.
 13. The power supply module according toclaim 12, wherein the first and second principal surfaces are parallelto and spaced from one another.